How can I make a surface look wet in cycles?
You need larger bumps. Using only the roughness from the glossy shader gives too smooth a result.

With a very quick setup:



The diffuse node represents the asphalt/road underneath the water. By mixing this with a glossy node based on the IOR of water (~1.33), it appears as if there is a thin film of specularly reflecting material (water) on top of a very bumpy diffuse material (road).



This could use a bit more large variation (cracks and large features, etc.), so you may want to use an image texture instead.